maybe they need to reinstall the XP?
or the video drivers?
bad video card or cables/connections or monitor?

my new motherboard has has built in video but I still had to load the drivers and it has a secondary program for many more settings/customizations than the XP settings in control panel.
but to get to those you still go to settings, control panel, display, settings and on that page go to advanced, then mine says the vid card name {GeForce6150} and that opens up all kinds of settings and options.

I don't know if that will help them
- how long have they had this computer system?

was it working well and then started acting up?
or a new system
or someone changed some settings?

If it worked fine before maybe try a system restore back to the time when it worked well?